A key feature of quantum physics is the wave-particle duality: the tendency of physical systems to exhibit both wavelike and particle-like behaviors. One particularly striking example of the wave-particle duality is the quantum eraser. In a typical experiment, two photons are entangled in a particular way and sent along different paths. As is usual in entanglement, a measurement performed on one photon reveals the outcome of related measurements on the second. In the specific case of the quantum eraser, however, the measurement dictates whether the second photon will exhibit wave- or particle-like characteristics.

Because quantum eraser experiments rely on entanglement, the impact of the measurement influences the second photon instantaneously. But to date, all the examples have been performed under circumstances that would technically allow communication between the devices that perform the measurements. New results by Xiao-Song Ma and colleagues definitively rule that possibility out: they placed the experimental apparatus on two of the Canary Islands, separated by 144 kilometers.

The quantum eraser experiment involves producing two sets of photons with correlated polarizations. One set, known as the system photons, are sent into a polarizing beam-splitter (PBS); as the name suggests, this directs light along different paths based on its polarization. The two possible paths for the system photon were then recombined, so they could either interfere (if the photon is behaving like a wave) or show up in one of two detectors (behaving like a particle).

In this case, the second group of photons—called the environment photons—was sent 144km across open air from La Palma to Tenerife. (The team had broken the previous record for entanglement across wide distances.) The lab on the second island used a telescope to collect the light (which dispersed significantly over the intervening distance) and send it to a device to measure its polarization. The orientation of this device was selected randomly using a "quantum random number generator."

In one orientation, the detector measured the circular polarization of the environment photons. Because they were entangled, the system photons also interacted as circularly polarized light, so the two paths produced by the PBS interfered with each other—meaning they behaved like waves. If the detector was set to the other orientation, it measured linear polarization of the environment photons. That meant the system photons also remained in linear polarization mode, so the PBS would simply act like a filter, sending the photon either along one path or another without interference. That selected the particle-like behavior for the system photons.

That's the nature of the quantum eraser: in the particle mode orientation, the interference pattern that would ordinarily occur was "erased." In wave mode, the specific path a photon might follow was erased. Since the distant detector on Tenerife was the one to select which mode the eraser would operate in, the result of the measurement on the system photons was known first—earlier in time than the "decision" was made. This is known as a delayed-choice measurement.

Partly due to the intrinsic difficulty of entanglement measurements, prior quantum erasure experiments were confined to a lab, meaning smaller distances between detectors. That meant experimenters could not rule out some kind of physical interaction between the detectors. The latest quantum eraser measurements, as with other long-distance entanglement experiments, handily eliminate the possibility of communication between the different detectors. This specific example just does so in a very intuitive manner.

This doesn't do violence to relativity by assuming instantaneous information transfer. Prior experiments ruled out the possibility of faster-than-light, yet non-instantaneous communication (see previous Ars coverage here). We've known for some years that entanglement precludes communication between apparatus separated by large distances.

Philosophical questions aside, the interesting aspect of quantum erasers (to this writer at least) is what they reveal about our preconceptions of particle and wave behavior.

No, this doesn't violate relativity. No information is transferred. You know what the result of the distant experiment will be, but the distant experimenter doesn't. To tell the distant experimenter you must send a signal, and that's still limited to the speed of light. Thus, they'll have completed their experiment before the signal arrives, and can merely confirm that you know the result.

No, this doesn't violate relativity. No information is transferred. You know what the result of the distant experiment will be, but the distant experimenter doesn't. To tell the distant experimenter you must send a signal, and that's still limited to the speed of light. Thus, they'll have completed their experiment before the signal arrives, and can merely confirm that you know the result.

I'm confused as well. If a 'quantum random number generator' was used why couldn't you just as easily use a known pattern? Otherwise isn't this experiment somehow testing the nature of your random number generator?

I thought this line was interesting: "This doesn't do violence to relativity by assuming instantaneous information transfer."

Why does everyone want to hurt relativity!!? If they wish to do so they will have to go through me first. NO WIMPS ALLOWED.

What about attacks by some MACHO MANs?

Jokes like this (and the others that abound in the field of physics) make me believe that the parts of a quantum physicist's brain concerned with humor are entirely subsumed by the parts concerned with their work, leaving untouched only those fully developed around age 10 or so.

No, this doesn't violate relativity. No information is transferred. You know what the result of the distant experiment will be, but the distant experimenter doesn't. To tell the distant experimenter you must send a signal, and that's still limited to the speed of light. Thus, they'll have completed their experiment before the signal arrives, and can merely confirm that you know the result.

So what happens when Alice&Bob can send the result to Victor because the route from the source to Victor is wrapped around the world. Victor then chooses to break the correlation. Then a black whole forms...

I thought this line was interesting: "This doesn't do violence to relativity by assuming instantaneous information transfer."

Why does everyone want to hurt relativity!!? If they wish to do so they will have to go through me first. NO WIMPS ALLOWED.

What about attacks by some MACHO MANs?

Jokes like this (and the others that abound in the field of physics) make me believe that the parts of a quantum physicist's brain concerned with humor are entirely subsumed by the parts concerned with their work, leaving untouched only those fully developed around age 10 or so.

I'm not even close to the field of quantum physics, but that's just the duality exhibited by their brains. Until they decide on what to focus, they are capable of amazing mental work and poor jokes.

It sounds like it to me. The reason that you can't send a message with entangled particles is that you can "discover" what the remote measurement is going to show, but not control it; you can't choose what state you get when you collapse the system, so you can't pick what state they'll get. But as described, that's exactly what they're doing here; the system that reads the environment photons *does* pick what state the entangled photons ultimately wind up in, and that choice is supposedly propagated to the system observing the lab photons. The article even specifically says that the remote system picks what state the lab system gets, "particle mode" or "wave mode," and that the lab system then sees that choice potentially before it's actually made at the environment end.

Either there's an error in the article, or this group did just create a relativity-violating faster-than-light quantum radio.

Edit: by "the article," I mean the Ars write-up, and not the paper itself.

I have a deck of playing cards. I take two cards out of the deck randomly. They are a 2 card and a 5 card. I put them face down and mix them up so that I couldn't possibly know which card is which. Are these two cards entangled now because if I flip one over and it is a 2, that influences the other card into being a 5? Or are they only entangled if I can manipulate the revealed 2 card into a 5 card and therefore the original 5 card, through manipulating the 2 card, becomes a 2 card itself?

Two other points: (1) If entanglement can produce a "teleporter" wouldn't it not really be teleportation? Only information would be ported anywhere. Your physical manifestation would be annihilated in one location and a copy would be constructed from the entanglement. And if relativity holds true, for a period of time you would no longer exist. (2) The article mentions long-distance experimentation. I assume this means in relation to similar experiments performed previously, not in relation to say the distance across the universe. In that case, the experiment is rather short-range isn't it?

I have a deck of playing cards. I take two cards out of the deck randomly. They are a 2 card and a 5 card. I put them face down and mix them up so that I couldn't possibly know which card is which. Are these two cards entangled now because if I flip one over and it is a 2, that influences the other card into being a 5? Or are they only entangled if I can manipulate the revealed 2 card into a 5 card and therefore the original 5 card, through manipulating the 2 card, becomes a 2 card itself?

Close. The reason entanglement is interesting is that (using your example), each card is both a 2 and a 5 at the same time before you flip one. It exists in both states simultaneously, until it is measured. Only then does it "settle" into one or the other. The problem is that we cannot flip the card in such a way to ensure that it is a 2 or a 5 so we cannot use it to force the other card to be any specific value.

The spooky part is that somehow the entangled objects always "align". Click the "delayed-choice experiment" link in the article for more info.

I have a deck of playing cards. I take two cards out of the deck randomly. They are a 2 card and a 5 card. I put them face down and mix them up so that I couldn't possibly know which card is which. Are these two cards entangled now because if I flip one over and it is a 2, that influences the other card into being a 5? Or are they only entangled if I can manipulate the revealed 2 card into a 5 card and therefore the original 5 card, through manipulating the 2 card, becomes a 2 card itself?

Warning, I am not a physicist.

As an idealized thought experiment, those two cards would be entangled. We've got two states for each card; it's either a two or a five. But, in the idealized case, after being shuffled face-down, we don't know which one's the two and which one's the five; we have no information on which one's which, and there're no physical constraints that would require one to be the five, or make one more likely to be the five. We therefore say that the cards are in a superposition of multiple states; they're each half-two-state and half-five-state.

What makes these entangled is that learning about one of the cards tells us about the other. When you flip one card over, you collapse its superposition; you discover it's either entirely in the two-state or entirely in the five-state. But because you know you had one two card and one five card, you also now know something about the other card; you've also collapsed its superposed state too, and it's now either all-five-state or all-two-state, whether you've actually turned it over yet or not.

You could have two cards both in a superposition like that, and have them not be entangled. Imagine if you have two decks, which for clarity I'll call A and B (but we need to require that both decks are identical, and we can't tell an A-card from a B-card). From deck A, you pull a two and five, turn them face down, and shuffle them up. Then, from the B deck, pull a two and a five card from it, turn them face down, and shuffle them together. Finally, throw an A card away, and throw a B card away. Then shuffle the A and B card together.

If you followed my poorly-constructed and confusing example, what you've now got, again, are two cards, each of which is in a half-two/half-five superposition. However, this time, they're not entangled; when you flip one card over, you'll collapse its superposed state and get either a two or a five, but you won't learn anything about the other card, since it came from a different pair and could still be either a two or a five with equal, unchanged probability.

I have a deck of playing cards. I take two cards out of the deck randomly. They are a 2 card and a 5 card. I put them face down and mix them up so that I couldn't possibly know which card is which. Are these two cards entangled now because if I flip one over and it is a 2, that influences the other card into being a 5? Or are they only entangled if I can manipulate the revealed 2 card into a 5 card and therefore the original 5 card, through manipulating the 2 card, becomes a 2 card itself?

Warning, I am not a physicist.

As an idealized thought experiment, those two cards would be entangled. We've got two states for each card; it's either a two or a five. But, in the idealized case, after being shuffled face-down, we don't know which one's the two and which one's the five; we have no information on which one's which, and they're no physical constraints that would require one to be the five, or make one more likely to be the five. We therefore say that the cards are in a superposition of multiple states; they're each half-two-state and half-five-state.

What makes these entangled is that learning about one of the cards tells us about the other. When you flip one card over, you collapse its superposition; you discover it's either entirely in the two-state or entirely in the five-state. But because you know you had one two card and one five card, you also now know something about the other card; you've also collapsed its superposed state too, and it's now either all-five-state or all-two-state, whether you've actually turned it over yet or not.

You could have two cards both in a superposition like that, and have them not be entangled. Imagine if you have two decks, which for clarity I'll call A and B (but we need to require that both decks are identical, and we can't tell an A-card from a B-card). From deck A, you pull a two and five, turn them face down, and shuffle them up. Then, from the B deck, pull a two and a five card from it, turn them face down, and shuffle them together. Finally, throw an A card away, and throw a B card away. Then shuffle the A and B card together.

If you followed my poorly-constructed and confusing example, what you've now got, again, are two cards, each of which is in a half-two/half-five superposition. However, this time, they're not entangled; when you flip one card over, you'll collapse its superposed state and get either a two or a five, but you won't learn anything about the other card, since it came from a different pair and could still be either a two or a five with equal, unchanged probability.

I really hope that helps.

I think you need a new perspective. If you have two cards entangled with each other, its really like saying we have 1 card in two positions. The outcome of whether one card is a 2 or a 5 is determined by if the other card is a 2 or a 5. But in your example, if you flip over one card and its a 2, then the other card would be a 2 as well. The trouble is that you can't think of them as 2 values, or 2 separate entities in any way except spatially.

I have a deck of playing cards. I take two cards out of the deck randomly. They are a 2 card and a 5 card. I put them face down and mix them up so that I couldn't possibly know which card is which. Are these two cards entangled now because if I flip one over and it is a 2, that influences the other card into being a 5? Or are they only entangled if I can manipulate the revealed 2 card into a 5 card and therefore the original 5 card, through manipulating the 2 card, becomes a 2 card itself?

Close. The reason entanglement is interesting is that (using your example), each card is both a 2 and a 5 at the same time before you flip one. It exists in both states simultaneously, until it is measured. Only then does it "settle" into one or the other. The problem is that we cannot flip the card in such a way to ensure that it is a 2 or a 5 so we cannot use it to force the other card to be any specific value.

The spooky part is that somehow the entangled objects always "align". Click the "delayed-choice experiment" link in the article for more info.

Yes. To continue with the set-up, one of the neat things about this is that the cards appear to "communicate".

Imagine that you've got your two face-down entangled cards. Also, now imagine that you have a buddy with a rocket, and that you give him a (face-down, un-peeked!) card, and that he then promptly flies away. A year or two of traveling later, he's light-months away; you then flip over a card, and discover that it's a two. You now know that he's got the five, without having to radio out and check. If he flips his card "immediately," he'll discover that he's got the five, even tho there was no time for a light-signal to reach him. The universe is somehow keeping all this straight, even tho it would've violated the speed-of-light signal for an "I've got the two" message to travel from you to your friend in time.

The reason this is O.K. by the speed-of-light limit is that you can't control which card you get. You can't arrange to have the two or the five; you only get an entangled pair at all if you're perfectly and completely ignorant about which card you got, and you can't retroactively make your card the two or the five after you and your buddy part ways. Therefore you can't use this weird phenomenon to send a message to your buddy.

And that's my and a few other people's problem with this article. As described, in this experiment, the remote system is somehow magically choosing whether it gets a two or a five, and having that choice show up in the lab system. If that really is what's happening in this experiment, then this group really has just experimentally demonstrated a speed-of-light violation, invented a superluminal quantum radio, and thrown relativity out the window.

I think you need a new perspective. If you have two cards entangled with each other, its really like saying we have 1 card in two positions.

An entangled pair is still a pair of entities. In this example, we do meaningfully have two cards with two states.

SgtCupCake wrote:

The outcome of whether one card is a 2 or a 5 is determined by if the other card is a 2 or a 5. But in your example, if you flip over one card and its a 2, then the other card would be a 2 as well. The trouble is that you can't think of them as 2 values, or 2 separate entities in any way except spatially.

If we entangle a two and a five card, then we should get a two and a five card when we're done, not two two cards or two five cards.

Are you saying that you think that you can't entangle entities with different states -- like we could entangle two cards in (equal) two/five superpositions, but not a two card and a five card?

An entangled pair is still a pair of entities. In this example, we do meaningfully have two cards with two states.

2 cards in two DIFFERENT states could not be entangled and retain their unique values. Most of the time, the simple act of measuring one card would make the other go away. You cannot entangle the cards and still considered them separate in any way. Period.(except spatially)

boldingd wrote:

SgtCupCake wrote:

The outcome of whether one card is a 2 or a 5 is determined by if the other card is a 2 or a 5. But in your example, if you flip over one card and its a 2, then the other card would be a 2 as well. The trouble is that you can't think of them as 2 values, or 2 separate entities in any way except spatially.

If we entangle a two and a five card, then we should get a two and a five card when we're done, not two two cards or two five cards.

Are you saying that you think that you can't entangle entities with different states -- like we could entangle two cards in (equal) two/five superpositions, but not a two card and a five card?

Yes, if you entangle the cards they share the same value.(because they are the same!) Either a 2, 5, or both.

The reason entanglement is interesting is that (using your example), each card is both a 2 and a 5 at the same time before you flip one.

call me retrograde,but it seems to me that the trouble comes in from insisting the card is both a 2 and a 5 simultaneously.i'd go back and look harder for hidden variables.

.. and i'd also note that the authors of the original paper are also suggesting we reconsider parts of our model of quantum mechanics.here are the last two sentences of the conclusion of the original paper:

Quote:

Our results demonstrate that the view point that the system photon behaves either definitely as a wave or definitely as a particle requires faster-than-light communication. Since this would be in strong tension with the special theory of relativity, we believe that such a view point should be given up entirely.

2 cards in two DIFFERENT states could not be entangled and retain their unique values. Most of the time, the simple act of measuring one card would make the other go away. You cannot entangle the cards and still considered them separate in any way. Period.(except spatially)

There's a problem here. Entanglement isn't magic or alchemy; if you entangle a two and a five, you have to get a two and a five out; physical states can't just transmogrify like that. Spin, for example, is conserved, and it can't just reverse. So you're either telling me that I can't entangle two particles in different states at all -- that by no experimental set-up could I take a two-card and a five-card and entangle them -- or you're telling me that I can entangle a two-card and a five-card, but when I do, I will either magically turn the two into a second five, or the five into a second two.

... the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description (state), which is indefinite in terms of important factors such as position,[5] momentum, spin, polarization, etc.

Which sounds kinda like what you're talking about. But skim down the page, to this:

A Wooki Quantum-Mechanic wrote:

An example of entanglement occurs when subatomic particles decay into other particles. These decay events obey the various conservation laws, and as a result, pairs of particles can be generated so that they are in some specific quantum states. For instance, a pair of these particles may be generated having a two-state spin: one must be spin up and the other must be spin down. This type of entangled pair, where the particles always have opposite spin, is known as the spin anti-correlated case, and if the probabilities for measuring each spin are equal, the pair is said to be in the singlet state.

If each of two hypothetical experimenters, Alice and Bob, has one of the particles that form an entangled pair, and Alice measures the spin of her particle, the measurement will be entirely unpredictable, with a 50% probability of the spin being up or down. But if Bob subsequently measures the spin of his particle, the measurement will be entirely predictable―always opposite to Alice's, hence perfectly anti-correlated.

Emphasis mine. Note that, in this example, you have two particles in two different states -- one is spin-up and the other is spin-down -- that become entangled with each other. When measurements are performed, you don't get two spin-]up or two spin-down particles; you get a spin-up particle and a spin-down particle.

call me retrograde,but it seems to me that the trouble comes in from insisting the card is both a 2 and a 5 simultaneously.i'd go back and look harder for hidden variables.

You're retrograde.

That's the standard interpretation of a superposition as far as I understand it, that the cards are simultaneously in both states.

elenzil wrote:

.. and i'd also note that the authors of the original paper are also suggesting we reconsider parts of our model of quantum mechanics.here are the last two sentences of the conclusion of the original paper:

Quote:

Our results demonstrate that the view point that the system photon behaves either definitely as a wave or definitely as a particle requires faster-than-light communication. Since this would be in strong tension with the special theory of relativity, we believe that such a view point should be given up entirely.

I'm becoming suspicious of this paper. Permit a stupid question, because I don't know how the arxiv works: has this actually survived peer-review and been published yet? Because in their own summary, it sounds like they admit that their result would imply a causality violation.

arXiv is pre-review, (ie not peer-reviewed)but it's also the de-facto place for contemporary serious physicists to publish their work,so i'd take papers there seriously.

Demonstration of "reverse cuasality" of the kind it sounds like they're claiming would be a big fucking deal, tho. I mean, incredibly huge. Because what they're claiming to have done sounds a whole lot like a time-traveling, faster-than-light quantum radio.

Edit: To be specific, "reverse causality" is a code-phrase for time travel; exactly what that describes is an an even preceding its cause.

2 cards in two DIFFERENT states could not be entangled and retain their unique values. Most of the time, the simple act of measuring one card would make the other go away. You cannot entangle the cards and still considered them separate in any way. Period.(except spatially)

There's a problem here. Entanglement isn't magic or alchemy; if you entangle a two and a five, you have to get a two and a five out; physical states can't just transmogrify like that. Spin, for example, is conserved, and it can't just reverse. So you're either telling me that I can't entangle two particles in different states at all -- that by no experimental set-up could I take a two-card and a five-card and entangle them -- or you're telling me that I can entangle a two-card and a five-card, but when I do, I will either magically turn the two into a second five, or the five into a second two.

... the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description (state), which is indefinite in terms of important factors such as position,[5] momentum, spin, polarization, etc.

Which sounds kinda like what you're talking about. But skim down the page, to this:

A Wooki Quantum-Mechanic wrote:

An example of entanglement occurs when subatomic particles decay into other particles. These decay events obey the various conservation laws, and as a result, pairs of particles can be generated so that they are in some specific quantum states. For instance, a pair of these particles may be generated having a two-state spin: one must be spin up and the other must be spin down. This type of entangled pair, where the particles always have opposite spin, is known as the spin anti-correlated case, and if the probabilities for measuring each spin are equal, the pair is said to be in the singlet state.

If each of two hypothetical experimenters, Alice and Bob, has one of the particles that form an entangled pair, and Alice measures the spin of her particle, the measurement will be entirely unpredictable, with a 50% probability of the spin being up or down. But if Bob subsequently measures the spin of his particle, the measurement will be entirely predictable―always opposite to Alice's, hence perfectly anti-correlated.

Emphasis mine. Note that, in this example, you have two particles in two different states -- one is spin-up and the other is spin-down -- that become entangled with each other. When measurements are performed, you don't get two spin-]up or two spin-down particles; you get a spin-up particle and a spin-down particle.

An interesting phenomena. However, if you did not notice these particles are not measured simultaneously. Alice, can only measure her particle once. For all we know, when Bob measure's his particle it changes the state of Alices's particle. The only thing being described here is that the measurement of Alice's particle will produce the opposite measurement of Bob's particle, but it does not demonstrate that either particle being measured does not have the same value simultaneously.

An interesting phenomena. However, if you did not notice these particles are not measured simultaneously. Bob, can only measure his particle once. For all we know, when Alice measure's her particle it changes the state of Bob's particle. The only thing being described here is that the measurement of Bob's particle will produce the opposite measurement of Alice's particle, but it does not demonstrate that either particle being measured does not have the same value simultaneously.

But notice that you've got two particles in two different states entangled together, and when you measure them, you see two different states. They're sharing a state in that they're both in an identical spin-up/spin-down superposition while unobserved, but they're still two particles, and they still get different eigenstates (one of the two eigenstates the system is superposed in) when you actually observe them. You don't get two spin-up or two spin-down particles, you get a spin-up particle and a spin-down particle, just like what was entangled.

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

> Demonstration of "reverse cuasality" of the kind it sounds like they're claiming would be a big fucking deal, tho. I mean, incredibly huge.> Because what they're claiming to have done sounds a whole lot like a time-traveling, faster-than-light quantum radio.

> Edit: To be specific, "reverse causality" is a code-phrase for time travel; exactly what that describes is an an even preceding its cause.

right. i'm aware what 'reverse causality' means,and i agree it's not a finding which should be accepted lightly.

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

The causal order of events separated by a spacelike interval is unspecifiable, unless they re-wrote all of relativity in the last few years.

Wikipedia wrote:

The interval AC in the diagram is 'space-like'; i.e., there is a frame of reference in which events A and C occur simultaneously, separated only in space. There are also frames in which A precedes C (as shown) and frames in which C precedes A. If it were possible for a cause-and-effect relationship to exist between events A and C, then paradoxes of causality would result. For example, if A was the cause, and C the effect, then there would be frames of reference in which the effect preceded the cause. Although this in itself won't give rise to a paradox, one can show[29][30] that faster than light signals can be sent back into one's own past. A causal paradox can then be constructed by sending the signal if and only if no signal was received previously.

Having said that, I just read through the paper linked above, and from what I understand, it does state that the space-like separated choice and measurement of the environment photon influences the observation of interference (or not) on the system photon.

Furthermore, as I understand it, "normal" entanglement of a single pair of photons doesn't allow transfer of information at FTL speeds, because one cannot control the state into which the entangled system collapses, and hence no information is transferred. It would appear that, as several commenters above already stated, that using this setup this is possible. Note that [edit] distinct properties of the photons are entangled, viz., the path of the system photon and the polarization of the environment photon.

What I'm trying to say is, there doesn't appear to be an error in the (Ars) article on this account. Furthermore, note that while the link posted in the comments refers to the paper as posted on arxiv, Ars' source is PNAS. So, re: peer-review, I would assume that these results have at least been looked at by now (the paper on arxiv is dated to June 2012).

This leads me to conclude that either a procedural error must have been made during these experiments, or both the authors of the paper and Ars are using some very ambiguous wording in their explanation. Alternatively, holy shit.

Regarding the whole reverse causality thing, and the possibility of causal paradoxes mentioned above, I'm curious; does the math say anything about what would happen if something like that would happen?

Regarding the whole reverse causality thing, and the possibility of causal paradoxes mentioned above, I'm curious; does the math say anything about what would happen if something like that would happen?

Well, to the photons themselves, there is no reverse causality since "time stands still" if one is moving at light speed. To those of us who aren't photons, it really sounds a lot like reverse causality.

There is a very straightforward way to test this: use a deterministic "random" generator. Also, it should be possible to send more than one signal during the flight time of the photons to rule out other confounding factors such as the system 's current state somehow affecting the environment detector.

BTW, I'm curious how they found 144km of straight line-of-sight on a curved Earth.

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

ngativ wrote:

boldingd wrote:

SgtCupCake wrote:

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

ngativ wrote:

boldingd wrote:

SgtCupCake wrote:

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

But sometimes events can appear to repeat within a short spacelike span.

Order-of-observation has nothing to do with it -- and to underscore that point, notice that order-of-observation is undefinable if the observation events are space-like-separated, which they are in this example.

The order of observation is not undefinable by spacing. On the contrary.

The causal order of events separated by a spacelike interval is unspecifiable, unless they re-wrote all of relativity in the last few years.

Wikipedia wrote:

The interval AC in the diagram is 'space-like'; i.e., there is a frame of reference in which events A and C occur simultaneously, separated only in space. There are also frames in which A precedes C (as shown) and frames in which C precedes A. If it were possible for a cause-and-effect relationship to exist between events A and C, then paradoxes of causality would result. For example, if A was the cause, and C the effect, then there would be frames of reference in which the effect preceded the cause. Although this in itself won't give rise to a paradox, one can show[29][30] that faster than light signals can be sent back into one's own past. A causal paradox can then be constructed by sending the signal if and only if no signal was received previously.